Durability and damage mechanism of carbon fiber–reinforced poly(ether-ether-ketone) composites (T300/PEEK) have been investigated under ultraviolet (UV) and water condensation conditions for 1560 h. The tensile modulus decreased by 5.4% after 1560 h of exposure, while no significant changes were found in tensile strength. The microhardness and elastic modulus of the resin measured by atomic force microscope–based nanoindentation were found to be dramatically increased after 240 h treatment and then decreased after longer treatment. The thermal decomposition temperature decreased from 549° to 522° after 840 h of exposure due to the formation of side chains and low molecular products induced by UV. The damage of resin was attributed to chain scission and recombined cross-linking by UV irradiation and hydrolytic deterioration by hydrothermal conditioning, where the decomposition led to the formation of carbonyl groups and hydroxyl groups, as well as the reduction of ether groups determined by Fourier transform infrared spectroscope. Scanning electron microscopy analysis on tensile fractures near the exposed surface indicated fiber/matrix debonding. The resin on the surface degraded rapidly, and its roughness increased continuously from 30.8 ± 4.1 nm to 88.8 ± 6.8 nm after 840 h of degradation, with the formation of microholes and microcracks. A degradation mechanism was proposed, and the accelerated weather aging affected only the surface region of T300/PEEK.